Crude petroleum contains impurities which include water, salts in solution and solid particulate matter that may corrode and build up solid deposits in refinery units; these impurities must be removed from the crude oil before the oil can be processed in a refinery. The impurities are removed from the crude oil by a process known as “desalting”, in which hot crude oil is mixed with water and a suitable demulsifying agent to form a water-in-oil emulsion which provides intimate contact between the oil and water so that the salts pass into solution in the water. The emulsion is then passed into a high voltage electrostatic field inside a closed separator vessel. The electrostatic field coalesces and breaks the emulsion into an oil continuous phase and a water continuous phase. The oil continuous phase rises to the top to form the upper layer in the desalter from where it is continuously drawn off while the water continuous phase (commonly called “brine”) sinks to the bottom from where it is continuously removed. In addition, solids present in the crude will accumulate in the bottom of the desalter vessel. The desalter must be periodically jet washed to remove the accumulated solids such as clay, silt, sand, rust, and other debris by periodically recycling a portion of the desalter effluent water to agitate the accumulated solids so that they are washed out with the effluent water. These solids are then routed to the wastewater system. Similar equipment (or units) and procedures, except for the addition of water to the oil, are used in oil producing fields to dehydrate the oil before it is transported to a refinery.
During operation of such units, an emulsion phase of variable composition and thickness forms at the interface of the oil continuous phase and the water continuous phase in the unit. Certain crude oils contain natural surfactants in the crude oil (asphaltenes and resins) which tend to form a barrier around the water droplets in the emulsion, preventing coalescence and stabilizing the emulsion in the desalting vessel. Finely divided solid particles in the crude (<5 microns) may also act to stabilize the emulsion and it has been found that solids-stabilized emulsions present particular difficulties; clay fines such as those found in oils derived from oil sands are thought to be particularly effective in forming stable emulsions. This emulsion phase may become stable and persist in the desalting vessel. If this emulsion phase (commonly known as the “rag” layer) does stabilize and becomes too thick, the oil continuous phase will contain too much brine and the lower brine phase will contain unacceptable amounts of oil. In extreme cases it results in emulsion being withdrawn from the top or bottom of the unit. Oil entrainment in the water phase is a serious problem as it is environmentally impermissible and expensive to remedy outside the unit. Also, it is desirable to achieve maximum coalescence of any remaining oil droplets entrained in the water continuous phase and thereby ensure that the withdrawn water phase is substantially oil free by operating the unit with the water continuous phase to be as dose as possible to the high voltage electrodes in the unit without resulting in shorting across the oil to the water. If, on the one hand, the emulsion phase gets too thick the dosage of the demulsifying agent must be increased; on the other hand, if the water continuous phase gets too high or too low, the water phase withdrawal valve at the bottom of the unit called a “dump valve” must be correspondingly opened or closed to the degree necessary to reposition the water phase to the desired level in the unit and for this purpose.
Processing crudes with high rag layer formation tendencies in current desalter configurations may cause poor desalting (salt removal) efficiency due to solids build up at the bottom of the vessel, and/or a solids stabilized rag layer leading to erratic level control and insufficient residence time for proper water/oil separation. Solids stabilized emulsion layers have become a major desalter operating concern, generating desalter upsets, increased preheat train fouling, and deteriorating quality of the brine effluent and disruption of the operation of the downstream wastewater treatment facilities.
Refinery sites which process high solids content crudes (characterized as containing more than 150 ppm inorganic solids) have the most pervasive problems with emulsion formation. Heavy crude oils and bitumens from Western Canada which contain elevated levels of small clay fines and other small solids are particularly prone to forming large volumes of highly stable emulsion. Examples of Western Canadian crude oils with emulsion forming tendencies include but are not limited to: Western Canadian Select (WCS), Cold Lake Blend (CLB), Access Western Blend, Albian Heavy and Seal Heavy.
The water content of the emulsion can range from 50 to 95% water with the balance being hydrocarbon (normally full range crude oil) and up to 5 weight percent inorganic solids. Precipitated asphaltenes, waxes and paraffins are also found at elevated levels in the emulsion (compared to the incoming crude oil) which combine with particulates (solids), to bind the mixture together forming a complex structure which is highly stable. Intractable emulsions of this kind comprising oil, water and solids make adequate separation and oil recovery difficult. Often, these emulsions arising from the desalter are periodically discarded as are other intractable emulsions and slop streams throughout the refinery. This results in expensive treating or handling procedures or pollution problems as well as the fact that useful crude oil is also lost with these emulsions and slop streams.
Emulsions must be separated into well-defined oil and water phases before they can be reintroduced to refinery process units (e.g. crude distillation, coker, etc.) or waste water treatment plant. These stable emulsions cannot be completely separated by heating and conventional gravity settling and require specialized separation equipment.
In most cases, complete separation of water from the oil is inhibited by the presence of an envelope of solid or semi-solid material in a thin-film layer around the surface of each individual water droplet. This material may be inorganic, for example as clay platelets, or silica or limestone particles, or it may be organic such as wax-like or bitumen-like particles. These inorganic and organic solids act as emulsion stabilizers. Furthermore, if the oil has a specific gravity approaching that of water and has a high viscosity, the difficulty of separating these types of oil emulsions is further compounded. The high viscosity greatly hampers the effectiveness of separation equipment. These stable emulsions cannot be completely separated by heating and conventional gravity settling and require specialized separation equipment.
U.S. Pat. No. 4,938,876 describes a process in which emulsions are rendered more amenable to gravitational and cyclonic separation by causing a portion of the normally water dispersed phase to flash into vapor by suddenly reducing pressure on the emulsion which has been heated by direct contact with superheated water and/or steam. The flashing action accompanying the reduction in pressure is stated to be extremely powerful even when only a small fraction, 10 percent by volume or less, of the dispersed phase is vaporized. The envelope around each droplet is thus shattered so the dispersed phase can be coalesced and separated by gravity, or enhanced gravity forces, when there is a sufficient divergence of specific gravity and a low viscosity. Suitable anti-emulsion chemicals are often added to prevent re-emulsification.
U.S. Pat. No. 5,882,506 (Ohsol) describes method for treating desalter rag layer emulsions, for the recovery of processable oil values by adding a sufficient amount of a light hydrocarbon diluent to the emulsion to lower its overall viscosity and to reduce the specific gravity of the oil phase. The diluted emulsions are subjected to flashing at emulsion-breaking conditions after which the oil is recovered from the various streams created in the flashing steps.
One of the most common industry practice is to separate the stable emulsion into separate water, oil and solids phases using 3-phase centrifuges (decanter centrifuges). The centrifuge separation is often enhanced with the use of chemical emulsion breakers, heating and/or depressuring the emulsion to facilitate the process. US 2012/0024758 (Love) proposes a technique in which the emulsion “rag” layer is withdrawn from the separator vessel at a rate that maintains the height of the emulsion layer approximately constant so as to permit withdrawal of the rag layer at a fixed level from the vessel. The withdrawn emulsion is then processed outside the vessel through a stacked disk centrifuge.
Currently practiced centrifuge separation approach has numerous reliability and cost drawbacks centering on the separation of the oil and water phases before they can be reintroduced to refinery process units (e.g. crude distillation, coker, etc.) or the waste water treatment plant. Problem areas include:                High cost associated with processing emulsion stream typically through a third party reprocessor outside of the refinery's direct operational control.        Historically poor mechanical reliability and time on stream for the centrifuge separation process.        The need to mitigate or recover vapor emissions as the emulsion is processed. in the centrifuge. Heating of the emulsion is often necessary to achieve the separation which further increases the volume of vapor emissions to handle.        Large volumes of recovered solids which must be disposed of as hazardous waste or further processed to allow their disposal by the Mobil Oil Sludge Coking Process in the coker1. 1The Mobil Oil Sludge Conversion process, or the MOSC process is described in U.S. Pat. Nos. 3,917,564, 4,874,505 and 5,009,767.        Batch operation which requires storage for separated oil, water and solids.        Frequent recycling or reprocessing of the material to achieve complete phase separation.        
Recovery of the separated oil phase often requires reintroduction to the crude preheat upstream of the desalter where the emulsion was originally formed. The returns may contain contaminants which tend to reform emulsion.
U.S. Provisional Patent Application Ser. No. 61/774,957, filed 8 Mar. 2013 (EM Family No. 2013EM063), describes an improved mode of desalter operation in which provides for withdrawal of a portion of the emulsion layer from the desalter vessel through one or more external withdrawal headers according to the thickness and position of the emulsion layer with the selected withdrawal header(s) being controlled by sensors monitoring the position and thickness of the emulsion layer. The withdrawn emulsion layer is then routed as such or with the desalter water effluent to a settling tank or directly to another unit for separation and reprocessing.
U.S. Provisional Patent Application Ser. No. 61/828,963, filed 30 May 2013 (EM Family No. 2013EM170), describes an improved mode of desalter operation in which provides for withdrawal of a portion of the emulsion layer from the desalter vessel for treating the emulsion layer withdrawn from the desalter vessel in order to separate it into its oil and water components along with any solids brought along with it. This treatment comprises diluting the withdrawn emulsion with added water or oil to destabilize the emulsion and permit its subsequent separation. The desalting method is operated by forming a settled water layer containing the dissolved salts with a settled supernatant, desalted oil layer and an intervening emulsion layer formed from the oil and the water. A portion of the emulsion is withdrawn through one or more withdrawal ports or headers and diluted with an added fluid, typically water or an added hydrocarbon feedstock, to destabilize the emulsion which is then separated, optionally with the aid of an electrostatic precipitator in a separator vessel which itself may be a desalter type vessel operating with a high voltage electric filed to facilitate the separation.